The instant invention relates to the field of MOS (metal-oxide-semiconductor) integrated circuits.
In such circuits, one often tries to measure a current, for example for causing an action in response to this measuring. One of the difficulties is to be able to measure the current without modifying it and while not being sensitive to physical fluctuations, such as temperature variations or variations in the manufacturing process of the integrated circuit.
An example of the problems that can be encountered will be illustrated in connection with a specific application in relation with FIG. 1. A power MOS transistor 1 of the DMOS-type is usually technologically realized by a very large number N.sub.p of identical cells in parallel. A given number N.sub.s of cells forming a MOS transistor 2 can be arranged in series with a measurement resistor r. The voltage across this resistor gives an evaluation of the current value in the MOS transistor 2 which receives the same gate voltage V.sub.g as the main MOS transistor. This current is normally proportional to the main current according to the ratio N.sub.s /N.sub.p. In fact, this is not true since the operation is impaired by the resistor r. V.sub.1 being the gate/source voltage of transistor 1 and V.sub.2 the gate/source voltage of transistor 2: EQU V1=V.sub.2 +rI.sub.2
In other terms: EQU I.sub.1 /N.sub.p g.sub.m =(I.sub.2 /g.sub.m +rN.sub.g I.sub.2)N.sub.S EQU I.sub.2 /I.sub.1 =(N.sub.s /N.sub.p)(1+N.sub.g rg.sub.m).sup.-1
In those equations, g.sub.m designates the transconductance of the DMOS transistors. It can be seen that when the voltage across resistor r is detected, in order to determine I.sub.2, it is not possible to directly deduce therefrom I.sub.1 since g.sub.m has to be known. It could be conceivable to take this correction factor into account and obtain a roughly linear relation when r is low. However, the transconductance term g.sub.m is sensitive to the variations of the manufacturing processes and to the temperature. A reliable detector is therefore not achieved.
The problem encountered in this specific case, and in numerous cases for measuring low currents, is to permit measuring without causing any perturbation and especially perturbations correlated with physical parameters liable to variations.
This problem has been studied for a long time and various solutions have been proposed in the art. More generally, if the current supplied by the MOS transistor 2 is considered as being generated by a current source I.sub.s, a first solution is illustrated in FIG. 2. It simply consists in connecting the current source to the inverting terminal of an operational amplifier 10, the non-inverting terminal of which is grounded, a feedback resistor R connecting the output of the operational amplifier to its inverting input. Thus, the current source I.sub.s can be considered as being connected to a virtual ground and its operation is not impaired by a measuring resistor. However, this circuit presents the drawback that the voltage at the output of the operational amplifier 10 has to be negative (-RI.sub.s); it is therefore necessary to provide a negative biasing voltage source for this operational amplifier, while, in the integrated circuits, one tries to minimize the number of supply sources.
Another circuit according to the prior art, more elaborated and designed to avoid having to resort to negative biasing voltage sources, is illustrated in FIG. 3. This circuit will not be described in detail. However, it will be noted that it comprises three stages of operational amplifiers. The amplifier A1 gives at its output an image of the voltage across resistor r1. The amplifier A3 gives at its output an image of the voltage across resistor r2. The amplifier A2 biases the gate of the transistor T1 so that both images are equal, that is, the voltage across resistor r1 is equal to the voltage across resistor r2. Due to this fact, the voltage across resistor r3 is proportional to the current in resistor r1. This circuit presents the drawbacks of being affected by the offset voltages and exhibits a relatively high time constant, of about 2 microseconds.
Thus, the instant invention aims at realizing a current measuring circuit permitting to avoid impairing the current that is detected and to obtain a measuring voltage substantially independent of the variations of temperature or any other parameter of the integrated circuit containing this measuring circuit.